The present invention relates to a device and a method for measuring characteristics of a superconductor such as repulsive displacements and repulsive forces with respect to a magnet.
A variety of mechanical components such as superconductor bearings, equipment and devices which utilize the repulsive force of a superconductor with respect to a magnet have been proposed in the art since high temperature superconductors were found. In actually designing and manufacturing such a mechanical component using the diamagnetism (the Meisner effect), it is necessary to quantitatively measure the repulsive displacement and repulsive force of the superconductor with respect to a magnet.
The following first and second methods for quantitatively measuring the repulsive force of a superconductor with respect to a magnet were published in Japan in the meeting of Cryogenic Engineering and Superconductor Society on Nov. 20, 1988. Further, the following third method was published in USA in Appl. Phys. Lett. 52(18), May 2, 1988, F. C. Moon et al.
In the first method, a superconductor specimen cooled lower than the critical temperature is levitated over a permanent magnet, and the height of the specimen is visually measured.
In the second method, a superconductor specimen suspended from a gravimeter is set over an electromagnet, and it is cooled by immersing it in liquid nitrogen. Under this condition, the levitation force of the superconductor specimen is measured by subtracting a weight of the specimen measured when an electric current is flown in the electromagnet from a weight of the specimen measured when no electric current is flown in the electromagnet, and also change of the levitation force in accordance with increase or decrease of the current flowing in the electromagnet is measured.
In the third method, the repulsive force is measured from the plastic deformation of a cantilever attached to a permanent magnet levitated over a superconductor.
In the above-described first method, the measurement is limited to a relatively short time, and is visually performed. Therefore, the results of measurement vary in accordance with the skill of the operators, that is, the results of measurement according to the first method have large errors and are low in reliability.
In the second method, it is necessary to measure the current flowing in the electromagnet and the weight of the superconductor specimen. This is relatively troublesome. In addition, the cooling liquid nitrogen permeates the superconductor specimen, as a result of which the measurement is lowered in accuracy.
Furthermore, in each of the above-described two methods, the measurement value includes the weight of the superconductor as a parameter. In order to obtain the repulsive displacement and repulsive force with respect to a magnet, it is necessary to measure the weight of the superconductor and to perform an arithmetic operation according to the weight thus measured. Thus, the measurement is rather troublesome, and low in accuracy.
In the third method, it is impossible to measure the effect of rotation of the superconductor.
By the way, superconductors are classified into two groups; a group of type I superconductor to which mainly pure metals belong, and a group of type II superconductor to which alloys, inorganic compounds, amorphous alloys and organic compounds belong.
A type I superconductor is perfect-diamagnetic and superconducting until the critical magnetic field is reached. A type II superconductor is perfect-diamagnetic until the lower critical magnetic field is reached, and it is not only superconducting but also normal-conducting when held between the lower critical magnetic field and the upper critical magnetic field; that is, in this state, it allows superconducting current while permitting a partial penetration of the magnetic field.
It has been known in the art that, when a type II superconductor is mixedly superconducting and normal-conducting, a kind of frictional force is induced by the magnetic flux passing through the superconductor. This is called "a suspension effect". For example, in the case where superconductor levitating over a magnet is mixedly superconducting and normal-conducting, even if the magnet is set above the superconductor, the superconductor will be held suspended in midair, and will not drop.
As was described above, with the type II superconductor which is mixedly superconducting and normal-conducting, a kind of frictional force is induced with the magnetic field. Hence, when such a superconductor is used for a mechanical part such as a bearing, then the frictional force would be a resistance against the operation of the mechanical part. Therefore, as for a type II superconductor, it is necessary to measure the range in which the superconductor is mixedly superconducting and normal-conducting, and the resistance provided when it is mixedly superconducting and normal-conducting.
For the type II superconductor, the fact that the superconductor is mixedly superconducting and normal-conducting under a predetermined condition and the suspension effect have been known as was described above; however, a method and apparatus for quantitatively measuring the range in which the superconductor is mixedly superconducting and normal-conducting (hereinafter referred to as "a mixedly superconducting and normal-conducting range", when applicable) and the mechanical resistance due to the magnetic flux pinning in the superconductor have not been proposed yet.
The state that the superconductor is mixedly superconducting and normal-conducting can be detected by measuring the susceptibility of a superconductor. However, the method cannot measure the repulsive displacement and repulsive force with respect to the magnet, or the mechanical force such as resistance due to the flux pinning in the superconductor.
Furthermore, the above-described three repulsive force measuring methods are merely able to measure the repulsive force of a superconductor with respect to a magnet. It goes without saying that the methods cannot measure the mixedly superconducting and normal-conducting range and the mechanical resistance due to the flux pinning in the superconductor.